Studies involve collaboration in the Laboratory on specific problems in carcinogenesis including the characterization of neoplastic cells and the study of tumor histogenesis. The aim of the present investigation is to define by high resolution and immunoelectronmicroscopy changes in natural killer cells and targeted tumor cells which correlate with natural killer cell activity. Large granular lymphocytes (LGL) have been identified with natural killer cell activity. OX-8, a monoclonal antibody which reacts with a cell surface antigen of LGLs and cytotoxic/suppressor T lymphocytes, has been used in immunoelectron microscopy of lymph nodes from nude rats which lack the T cells. A postembedding staining procedure was used which gave moderate preservation of lymphoid cell structure, but poor preservation of nonlymphoid cells. Cells which stained positively with the antibody displayed characteristics consistent with their being LGLs (kidney shaped nucleus, cytoplasmic granules). Further investigations of the interactions between LGLs and tumor cells in vivo and in vitro will follow establishment of a protocol which gives good ultrastructural preservation and specific antibody staining.